The reason for the weird naming (that always confuses newcomers to SharePoint development) is backward compatibility. Back in SharePoint 2003 you had Microsoft SharePoint Portal Server 2003 as one product and Windows SharePoint Services as another (free) product. In WSS a sub-site was called a web and SPWeb in the object model. To make confusion even bigger, in Portal Server it was called an Area. The two object models were merged in SharePoint 2007 but due to backwards compatibility the naming was kept. Be aware that there also are SPWebCollections and SPSiteCollections
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Anders RaskMay 14 '10 at 12:17

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Just to be accurate it's SPSite and SPWeb (no plurals). 'Web' referring to sub sites is remnants from SharePoint 2003 terminology and it's not going anywhere because it's made it's way into SP2010 too. And just so we're all totally confused, there's SPSiteCollection which is a collection of site collections. Oh, and it's not just publishing sites.
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Anthony KassesMay 14 '10 at 12:25